This invention relates to a novel class of vapor sensors with tunable properties. More particularly, this invention relates to vapor sensors modified by the addition of a compatible small molecule of low volatility, i.e., a plasticizer.
There is considerable interest in developing chemically sensitive sensors that are capable of detecting the presence of a particular chemical analyte in a fluid. Because the fluid is typically air, such sensors act as electronic noses, xe2x80x9csmellingxe2x80x9d the presence of a particular airborne molecule. These sensors are often fabricated from a polymeric organic material that is capable of absorbing a chemical analyte which comes in contact therewith, wherein absorbance of the analyte causes the polymeric material to expand or change, thereby modifying the electrical properties of the sensor. Variability in the ability to absorb an analyte results in variability in the detectable signal produced. Such organic polymer-based sensors have found use in a variety of different applications and devices including, for example, devices that function as analogs of the mammalian olfactory system (see, U.S. Pat. No. 5,571,401, which issued to Lewis et al, Lundstrxc3x6m et al., Nature 352:47-50 (1991) and Shurmer and Gardner, Sens. Actuators B 8:1-11 (1992)), bulk conducting polymer films (Barker et al., Sens. Actuators B 17:143 (1994) and Gardner et al., Sens. Actuators B 18:240 (1994)), surface acoustic wave devices (Grate et al., Anal. Chem. 67:2162 (1995), Grate et al., Anal. Chem. 65:A987 (1993) and Grate et al, Anal. Chem. 65:A940 (1993)), fiber optic micromirrors (Hughes et al., J. Biochem. and Biotechnol. 41:77 (1 993)), quartz crystal microbalances (Chang et al., Anal. Chim. Acta 249:323 (1991)) and dye impregnated polymeric coatings on optical fibers (White et al, Anal. Chem. 68:2191 (1996)). To date, however, many of the sensors employed in the above-described devices have been fabricated from limited numbers of polymeric components and, therefore, are limited in the responses they are capable of producing.
Further, today""s technology lags far behind the ability of canines or humans to detect or distinguish between chemical analytes. As a consequence, certain work is limited by the suitability of animals or humans to execute tasks. For example, quality control of food products can require production line employees to smell each item. Unfortunately, the ability of individuals to adequately discriminate odors diminishes after a short period of time, e.g., in about two hours. In addition, mammalian olfactory senses are limited in their ability to identify certain vapors. For example, water vapor is not detectable by smell. FurthFurther, mammalian olfactory senses are limited to identifying gaseous components, with no ability to identify or xe2x80x9csmellxe2x80x9d solutes in liquids.
Recent studies have shown that arrays of chemically sensitive sensors, such as those disclosed in U.S. Pat. No. 5,571,401, formed from a library of expandable insulating organic polymers containing a conductor such as carbon black, are broadly responsive to a variety of analytes, yet allow classification and identification of organic vapors through application of pattern recognition methods. (Lonergan et al., Chem. Mater. 8:2298 (1996)). To date, these array elements have been fabricated from a relatively small number of approximately 10-20 organic polymers, with a single distinct polymer backbone composition in each sensor element. Although a limited number of polymeric sensor compositions might be chosen to perform optimally for specific applications, attempts to perform complex applications, such as to mimic the sense of olfaction, in which the sensing task is time dependent or is not defined in advance of the sensor array construction, will almost certainly require use of polymeric sensor libraries that are far more extensive and compositionally diverse than those presently known.
In general, plasticizers are organic compounds added to polymers to facilitate processing and to increase the flexibility and toughness of the polymeric product. Among the more important plasticizers are nonvolatile organic liquids and low melting solids such as phthalates, adipate and sebacate esters, polyols such as ethylene glycol and tricresyl phosphates.
U.S. Pat. No. 4,948,490, which issued to Venkatasetty, discloses a single cell electrochemical sensor utilizing a conducting polymeric solid electrolyte film. These conducting polymeric films such as polyethylene oxide, polypropylene oxide and polyvinylidine fluoride can be used with a plasticizer. A preferred plasticizer is polyethylene glycol dimethyl ether. The plasticizer is added to the mixture to increase ionic conductivity by converting some or substantially all of the structure from crystalline to a plasticized amorphous form.
In addition, U.S. Pat. No. 4,587,101, discloses a fluorescence oxygen sensor having a plasticized polymer with fluorescent indicator molecules embedded within the polymer. In operation, the presence of the oxygen reduces the intensity of the fluorescent indicator substance, thus facilitating detection.
European Patent Application No. 0 794 428, published Sep. 10, 1997, describes sensors capable of distinguishing between enantiomers. The sensor comprise a pair of spaced apart contacts and a conducting polymer material spanning the gap. The polymer has chiral sites in the polymer material formed by incorporating optically active counter ions such as camphor sulfonic acid.
WO 99/00663, published Jan. 7, 1999, the contents of which are incorporated by reference herein, discloses a sensor in which at least a first and second organic polymer are combined to form an organic polymer blend. The sensor will preferably provide a signal that is not linearly related to the mole fraction of at least one of the organic polymers used to produce the organic polymer blend.
In view of the foregoing, and despite the advances disclosed in WO 99/00663, there still remains a need for novel methods for producing large libraries of radically sensitive sensors having tunable properties, each of which are capable of producing a detectable response in the presence of an analyte of interest. The present invention fulfills this and other needs.
In accordance with the present invention, a novel organic polymer-based sensor is provided with tunable properties capable of detecting the presence of an analyte in a fluid, thereby providing a detectable response. As used herein, the term xe2x80x9cfluidxe2x80x9d includes gases, vapors, solids, and liquids. As used herein, the term xe2x80x9ctunablexe2x80x9d refers to tailoring a sensor to a specific application with specific types of polymers and constituents thereof. In certain embodiments, the sensor comprises an organic mixture and a detector operatively associated with the mixture. In one embodiment, the organic mixture comprises a compatible molecule of low volatility (a plasticizer) combined with an insulating organic polymer or a conducting organic polymer. In another embodiment, the organic mixture comprises a plasticizer combined with an organic polymer blend of a first organic polymer and a second organic polymer. In certain embodiments, the first or second polymers are both insulating polymers. In still another embodiment, the organic mixture comprises a plasticizer combined with an organic polymer formed from a first organic monomer and a second monomer. In yet another embodiment, the organic mixture comprises a plasticizer combined with an interpenetrating network comprising a first organic polymer and a second organic polymer formed from an organic monomer polymerized in the presence of the first organic polymer.
In another embodiment of the present invention, an electrically conductive material, which can be a single electrically conductive material or a mixture of two or more electrically conductive materials, is added to the organic mixture, such as an insulating organic polymer. In a preferred embodiment, the electrically conductive material is carbon black. In certain instances, the sensors are arranged in regions of conducting material and insulating material, such as in a matrix. In such embodiments, when the electrically conductive material is added to the organic mixture, the resulting sensor has a first electrical response in the absence of an analyte and a second electrical response in the presence of an analyte. In certain instances, the detector is an electrical measuring device electrically coupled to the sensor to measure the first and second electrical responses. Other suitable responses that can be detected include, but are not limited to, electrical responses, such as resistance, impedance, capacitance, optical response, magnetic response, surface acoustic response and fluorescence response.
In yet another embodiment of the present invention, sensor arrays are constructed with at least a first and a second sensor wherein each sensor comprises a polymer, such as an organic polymer (e.g., insulating or conducting). At least one of the first and second sensors includes a plasticizer combined with the polymer. A detector is operatively associated with each sensor. In a preferred embodiment, an organic material is mixed with an electrically conductive material. In such embodiments, the detector is an electrical measuring device electrically coupled to the sensor to measure the first and second electrical resistances.
In still yet another embodiment of the present invention, sensor arrays are constructed with at least a first and a second sensor wherein each sensor comprises a polymer, such as an organic polymer, such as an insulating organic polymer. At least one of the first and second sensors includes a plasticizer combined with the polymer. A detector is operatively associated with each sensor. In such embodiments, the detector is an electrical measuring device electrically coupled to the sensor to measure the first and second electrical responses.
Methods of using the sensors are also provided. One embodiment is a method for detecting the presence of an analyte in a fluid, the method includes, providing a sensor array comprising a plurality of sensors wherein each sensor in the plurality comprises a polymer, such as an organic polymer, and wherein at least one sensor in the plurality further comprises a plasticizer mixed with the polymer. A detector is operatively associated with each sensor. The detector can detect variations in, for example, electromagnetic energy, optical properties, resistance, capacitance, inductance or impedance of a combination thereof, and other physical, chemical and electrical properties that can vary in accordance with the response of the sensors. The sensors are then exposed to a fluid containing an analyte and the sensor responses measured. The sensor responses are compared to determine the presence of an analyte in the fluid. In a preferred embodiment, the polymer in the sensors includes an electrically conducting material, preferably carbon black, wherein the sensor response to the presence of the analyte, is a change in a resistance associated with the sensor. In such embodiments, the detector is an electrical measuring apparatus electrically coupled to the sensors, measuring an electrical response of each sensor. These and other embodiments will become more apparent when read with the accompanying detailed description of the invention.